An over-current protection device detects a current flowing through a load such as a three-phase motor via a contactor, and shuts off the current flowing to the motor when the current exceeds a safe threshold value. Conventionally, such a device is provided with a bi-metal switching element, and a part or all of the current to the motor flows through the bi-metal switching element. That is, the current flows though a switch consisting of the bi-metal element so that the bi-metal element is heated according to an intensity of the current. When the motor current exceeds a sate threshold value for a period of time longer than a predetermined time, the bi-metal element bends due to the heat to hold a switch contact in an open state, thereby shutting off the power to a control input terminal of a contactor. However, in the device using the bi-metal switch, it is difficult to adjust the current in the state that the switch is opened, so that the incorrectly adjusted condition tends to continue for a long time.
On the other hand, when an electric device is used instead of the bi-metal element, it is possible to electronically perform the function of the bi-metal switch. Accordingly, it is possible to improve reliability and easily adjust the device. However, the electronic device includes a complex circuit, and in order to properly detect a current to operate a contactor, it is necessary to provide a constant-voltage power supply and a large number of components. In addition, a current detection transformer has been used as a device for detecting current. Accordingly, it is difficult to obtain a wide range for detecting a current due to magnetic saturation of an iron core. It is possible to provide a magnetoresistive element as a device for detecting a current. However, it is necessary to provide an iron core due to a low sensitivity of the magnetoresistive element. Accordingly, similar to the current detection transformer, it is difficult to obtain a wide range for detecting a current.
To solve these problems, as a high sensitive magnetism detection element for replacing a Hall element and the magnetoresistive element, a magnetic impedance element using an amorphous wire has been disclosed (refer to Patent Document 1) Further, an amorphous magnetic thin film formed via a sputtering method has been used (refer to Patent Document 2).
When one of the magnetic impedance elements is used, it is possible to obtain high sensitivity in the magnetism detection characteristic. However, as shown in FIG. 17, impedance changes non-linearly relative to a magnetic field of the amorphous wire element, so that the magnetic impedance element has a non-linear output of the magnetism detection characteristic (refer to Patent Document 3). Accordingly, the linear output relative to the magnetic field is obtained from a difference in a variation of the magnetic impedance element obtained from a sum of the positive and negative magnetic fields generated by the AC bias magnetic field and the immeasurable external magnetic field, so that an AC bias magnetic field is applied to the magnetic impedance element (refer to Patent Document 4).
[Patent Document 1]
    Japanese Patent Publication (Kokai) No. 06-281712    (page 4, FIG. 5 to FIG. 12)[Patent Document 2]    Japanese Patent Publication (Kokai) No. 08-075835    (pages 4 to 5, FIG. 1 to FIG. 6)[Patent Document 3]    Japanese Patent Publication (Kokai) No. 2000-055996    (page 3, FIG. 23)[Patent Document 4]    Japanese Patent Publication (Kokai) No. 09-127218    (pages 4 to 5, FIG. 3)
Incidentally, in principle, the magnetic impedance element generates a magnetic impedance effect. Accordingly, it is necessary to apply a high-frequency current of several mA and at least several MHz to the element, thereby increasing the power consumption and a size of the power-supply transformer, and making it difficult to downsize the device and reduce cost of the device.
FIG. 16 shows an example of a conventional detecting circuit using the magnetic impedance element. The circuit includes sheared oscillating means 31 and bias-current applying means 13a1. A current of several mA and a bias current of about several tens of mA are constantly applied to magnetism detection elements 1a, 1b, and 1c, thereby increasing the power consumption in proportion to the number of the elements. Further, it is necessary to provide wave detection means 6a1, 6b1, and 6c1; holding means 8a1, 8b1, and 8c1; and amplifying means 11a1, 11b1, and 11c1 in proportion to the number of the magnetism detection elements, thereby increasing la size of the circuit and cost of the parts.
In view of the problems, the present invention has been made, and an object of the present invention is to provide an over-current protection device with a compact and low cost configuration having a low-cost power-supply source in which a constant-voltage regulated power-supply is not necessary. Further, it is possible to obtain a wide range of the current detection.